Active Matrix/Organic Light Emitting Diode (AMOLED) is so called next generation display technology. The AMOLED possesses fast response speed, wide view angle, high contrast, et cetera in comparison with traditional TFT-LCD. The driving of OLED requires higher mobility for the backplate TFT. At present, the mobility of the developed amorphous silicon (a-Si) merely can reach up to 0.5˜0.8 cm2/Vs, which cannot satisfy the normal driving of OLED. The Low Temperature Poly-silicon (LTPS) and the metal oxide semiconductor (such as IGZO) with higher mobility can be the research point for the thin film transistor (TFT) of the active layer.
In AMOLED, the OLED is driven by the thin film transistor (TFT) in the backplate. The backplate TFTs can be categorized as the Switching TFT and the Driving TFT. The Switching TFT is only for realizing the switching function. Thus, TFTs of low price and stable process can be utilized. However, the Driving TFT demands enough mobility for realizing the driving of the OLED. The TFTs with larger mobility can be utilized therefor.
According to the traditional OLED backplate process at present, the same structure and process order are employed for the Switching TFT and the Driving TFT, which the active layer utilizes only one kind of material (metal oxide semiconductor or LTPS). It is difficult to achieve the differentiations of the two kinds of TFTs of different functions. On the other hand, the Switching TFT, the Driving TFT and the capacitor in one pixel are independent from one another, which is a disadvantage for increase of the aperture ratio.
A common 2T1C structure (two TFTs and one storage capacitor) of driving OLED is shown in FIG. 1: the two TFTs 101, 102 are both the etching stopper layer (ESL) structure. The active layers 201, 202 utilize one kind of material (the metal oxide semiconductor or the LTPS). Totally, five masks are required for accomplishing the passivation layer 600. Meanwhile, after the same manufacture process, the TFTs 101, 102 possess the same electrical properties.